屬性觀察者何時應該運行？什麼是例外？

Question

在以下情況下，屬性觀察者的行爲讓我感到驚訝。我明白觀察者不會遞歸地調用自己，但是這種行爲似乎繼承了同一類的不同實例，甚至是不同子類的實例。

據我所知，屬性觀察者在任何時候都會運行一個屬性，即使這個值沒有改變，除了初始化。這個規則的例外究竟是什麼？如何看待財產觀察員究竟會被忽略？

var observersCalled = 0 

class ClassOne { 
    var relatedOne: ClassOne? 
    var relatedTwo: ClassTwo? 
    var property: String = "Initial" { 
     didSet { 
      observersCalled += 1 
      relatedOne?.property = property 
      relatedTwo?.property = property 
     } 
    } 
} 

class ClassTwo { 
    var property: String = "Initial" { 
     didSet { 
      observersCalled += 1 
     } 
    } 
} 

class Subclass: ClassOne { 
    override var property: String { 
     didSet { 
      observersCalled += 1 
     } 
    } 
} 

let thing = ClassOne() 
thing.relatedOne = ClassOne() 
thing.property = "New Value" 
print(observersCalled) //1 (really?) 

observersCalled = 0 
thing.relatedOne = nil 
thing.relatedTwo = ClassTwo() 
thing.property = "Another Value" 
print(observersCalled) //2 (makes sense) 

observersCalled = 0 
thing.relatedOne = Subclass() 
thing.relatedTwo = nil 
thing.property = "Yet Another Value" 
print(observersCalled) //1 (really!?) 

Answer 1

沒有例外。我問到的行爲是一個錯誤。我已提交a bug report追蹤其進展的人。